The ability of liquid crystal materials to change optically under the influence of an electric field makes them useful for a variety of applications, such as light valves, displays, etc. One prior art application of electrically controllable dynamic scattering materials employs a structure which is a cell of sandwich configuration comprising a front set of transparent planar electrodes and a specularly reflective back electrode spaced from the front electrode system. Between the two electrode systems is located a layer of active nematic material. With no electric field applied between the two electrode sets, the liquid crystal material is optically traansparent. Thus, if the back electrode reflects a black background, the cell looks black to an observer looking into it through its transparent front electrode system. However, when an unidirectional or alternating electric field is applied between the two spaced electrode systems, the liquid loses its transparent characteristic, scattering any light flowing into it through its transparent front electrode system. The scattered light is returned to the observer and the apparent color of the cell is generally of the same spectral content as the light passing into it through the front electrode system, i.e., nearly white in the usual circumstance. When the electric field is removed, the material reverts to its transparent state and looks black to the observer.
The scattering effect in these prior art devices in the presence of an electric field has been explained by ions set in motion as electrical current which pass through the normally aligned nematic medium thereby producing shearing disruptive forces. The turbulence thus produced causes localized variations in the effective index of refraction of the medium which in turn scatter the light passing through the cell.
In U.S. Pat. No. 3,807,831, issued to Richard A. Soref on Apr. 30, 1974, the patentee suggested the use of a liquid crystal cell comprising a liquid crystal layer and a pair of interleaved electrode sets on one side of the liquid crystal layer with polarizers oriented either parallel or perpendicular to one another on the two sides of the liquid crystal device. When an electric field is applied between adjacent pairs of electrodes the individual molecules reorient thus producing a birefringence in the liquid crystal layer. Depending upon whether the polarizers are oriented parallel or perpendicular to one another, the optical transmission of the cell will either decrease or increase in those portions of the cell where the electric field is applied. In Soref's patent the liquid crystal layer thickness varied from about 0.5 micron to about 1.5 microns and the aspect ratio of liquid crystal layer thickness to the interelectrode spacing varied from about 0.04 to about 0.15. Soref does not teach the use of such a device as a lens.
In U.S. Pat. No. 3,981,559, issued to Donald J. Channin on Sept. 21, 1976, the patentee suggested the use of a liquid crystal cell comprising a liquid crystal layer, a pair of interleaved electrodes on one side of said layer, a planar electrode on the opposed side of said layer, voltage means to apply an electric field between the pair of interleaved electrodes, and voltage means to apply an electric field between one of the pair of interleaved electrodes and said planar electrode. Channin did not teach the use of this cell as a lens in conjunction with other display elements to form a display system.